The three most common types of fluid filling schemes are volumetric filling, time-metered filling, and weight-metered filling. All are commonly implemented in semi-automatic or automatic filling systems wherein empty containers are presented by conveyors or other transport mechanisms to filling stations. Once the containers reach the filling stations, they are stopped, filled to the desired degree by nozzles or other dispensing apparata, and then released upon completion of the fill.
In volumetric filling (also known as volume-metered filling), a set volume of fluid is dispensed into a container: a chamber is set to a desired volume, the chamber is filled with product, and the contents of the chamber are then dispensed into a container. Volumetric filling is subject to the disadvantages that filling accuracy is limited by the accuracy of the control of the chamber volume, and filling speed is limited by the time necessary for refilling the chamber. Volumetric filling is also unsuitable where one wishes to fill a container with a desired weight of product: variations in product density will lead to variations in the weight of the product dispensed from the chamber and result in different weights being dispensed into different containers; viscous products may stick to the dispensing apparatus and result in incomplete dispensation; and so forth.
In time-metered filling (also known as time-metered volumetric filling), product is dispensed from a nozzle having a known volumetric flow rate for a set amount of time sufficient to fill the containers with a set volume of product. Time-metered filling is advantageous in terms of productivity insofar as one may reduce filling time per container to any desired level so long as the appropriate volumetric flow rate is obtainable. However, time-metered filling is subject to inaccuracy unless a constant flow rate is precisely maintained, and this is particularly difficult to attain where flow rates are high. Additionally, time-metered filling is subject to the same disadvantages as volumetric filling in that variations in product density will result in different weights of product being dispensed to different containers, even if the volume of the dispensed product remains relatively constant from container to container.
Weight-metered filling utilizes a weight sensor which monitors the amount of fluid received by a container. The weight sensor provides feedback to the dispensing apparatus, which halts dispensation when a desired weight of product is received. Weight-metered filling can be more accurate than volume-metered and time-metered filling, but it unfortunately has several significant disadvantages. First, the weight sensors and feedback apparata are quite costly if any reasonable degree of accuracy is required. Second, the filling time per container tends to be significantly longer owing to the weight feedback; sensitive weight sensors need time to "settle" prior to giving accurate weight readings, and additionally slower filling rates must often be used since the flow must be cut off precisely at or slightly before the time the desired weight is reached, or overshoot will result in an overweight container with product "give-away".
To address the shortcomings of these individual filling schemes, prior inventors have developed "hybrid" filling systems which use a combination of schemes in an attempt to attain better filling accuracy and/or higher product throughput. Initially, prior systems exist wherein time-metered filling and weight-metered filling are both used. U.S. Pat. No. 4,208,852 to Pioch describes a filling unit which forms, fills, and seals containers (e.g., column 1 lines 51-57). With reference to FIG. 1 of that patent, a container producing station 10 produces two containers at a time by use of two molds 20 (e.g., column 3 lines 10-29). The two containers are then indexed to a filling station 12, which includes four filling nozzles (e.g., column 4 lines 46-64; column 5 lines 43-62). The first two filling nozzles utilize time-metered filling to initially fill the two containers simultaneously, and the second two filling stations, which incorporate weight sensors, use weight-metered filling to complete the filling of the two containers to a desired weight. Thus, weight-metered filling is used for "topping off" the containers to a desired weight after time-metered filling, thereby correcting any inaccuracies in the time-metered filling. Closing stations 13 and 14 then close the containers.
Several systems are similar to Pioch in that they use time-metered filling with subsequent weighing of filled containers, but they do not use weight-metered filling. Instead, they use weight sensors to weigh the containers after they are filled, and the weight sensors then supply a correction signal to the time-metered filling stations if appropriate (i.e., they use weight sensors to "check-weigh" time-filled containers).
Examples of such systems are provided in the following patents.
U.S. Pat. No. 4,696,329 to Izzi describes a time- or count-metered filling unit wherein the containers are weighed after filling. After a first batch of containers is filled by a time-metered filling station, the weights of the containers in this first batch are averaged to derive a time correction signal for later containers if there is deviation between the measured weight and the desired weight (e.g., column 5 lines 51-60). If the containers are underweight, filling time is increased; if the containers are overweight, filling time is decreased; and if the container weights are within a preset tolerance band, filling time is left constant. Later batches of containers are similarly weighed to update the time correction for further containers. The weight sensor used to measure the container weights is located off-line of the conveyor that carries the containers beneath the time-metered filler (i.e., the weight sensor is not located at a filling station; see, e.g., column 4 lines 52-58).
U.S. Pat. No. 5,156,193 to Baruffato et al. describes a filling system wherein a weight sensor determines the tare weight of a container, the container is filled at a filling station via time-metered flow, and then the filled container is then finally weighed so that deviation from the desired weight can be used to modify the filling time (column 6 line 23-column 7 line 3).
U.S. Pat. No. 5,285,825 to Townsley describes a device for filling containers wherein containers are weighed at a first weighing station to obtain a tare weight, filled at first and second filling stations by time-metered filling (e.g., column 8 lines 3-13), and then weighed at a final weighing station to get the filled weight. Corrections to the fill time for subsequent containers are supplied to the filling nozzles based on average values of tare weight and gross weight for some number of containers.
U.S. Pat. No. 5,159,959 to Bohm is similar to the Izzi, Baruffato et al., and Townsley filling schemes, but it uses volumetric filling rather than time-metered filling. Volumetric charges of material are delivered to containers, and the containers are then weighed. Weight offsets (i.e., deviations from the desired final weight) are used to generate a correction signal which appropriately adjusts the volumetric chamber to produce a charge having the correct weight. As in Izzi, the weight offsets and correction signal may be generated from a number of filled packages rather than a single one so as to produce an "average" correction signal. A good summary of the invention is provided at column 2, line 12 onward.
There are also filling schemes similar to those of Izzi, Baruffato et al., and Townsley, but wherein weight sensors are provided at the time-metered filling stations rather than at separate locations. U.S. Pat. No. 5,109,894 to McGregor uses a weight sensor to support a container and fills the container by time-metered filling (more specifically, by counting revolutions of a dispensing auger). The weight sensor measures the weight of the filled container and adjusts the filling time if the desired product weight is not obtained (e.g., column 7 line 52-column 8 line 29).
U.S. Pat. No. 5,083,591 to Edwards et al. describes an automated system for dispensing volume- and weight-metered amounts of pigments and paint base into containers to obtain paint having a desired final color. As noted at column 3 line 53-column 4 line 18, the apparatus may include one or more filling stations, each having a weighing platform whereupon a container is placed during filling. The container is volumetrically filled. The weight sensor then determines whether the container is overweight or underweight; if it is underweight, more paint is added (e.g., column 20 lines 24-60), and if it is overweight, the controller will try to adjust the blending formula so that the proper paint color will still be obtained after subsequent filling steps are completed. Each filling station has a plurality of nozzles (each nozzle for a different pigment or base), and they may additionally be provided with multiple weight sensors, each sensor having a different tolerance so that addition of materials in different weight ranges can be more accurately metered (e.g., column 23 lines 12-44).
One system is known which provides weight-metered filling with a sort of time feedback (as opposed to the systems of Izzi, Baruffato et al., etc. above, which use time-metered filling with weight feedback). U.S. Pat. No. 5,148,841 to Graffin describes a filling unit with multiple filling nozzles for filling multiple containers. The nozzles are supplied with product from a reservoir, and each nozzle is controlled by a weight sensor which monitors the fill weight of its container and terminates flow when a desired weight is reached (e.g., column 3 lines 7-21). As noted at column 3 lines 40-58, timers are included to monitor the time needed to achieve the proper weight in each container, and the measured times are used to control the amount of material in the reservoir so that it has the desired dispensing pressure for future containers, thus establishing desired flow rates among the nozzles. As noted in columns 1 and 2, such desired flow rates may be chosen to avoid foaming, to provide an initial rapid fill rate and a later dribble fill rate for topping off containers, etc. Additionally, as noted at column 5 lines 26-38, this system allows the reservoir's product level to be adapted to provide a constant desired flow rate to each nozzle when one or more nozzles are either placed in service or out of service. Thus, filling of containers is ultimately done by weight (the weight error signals are not used to alter product flow rates), and time measurements are used to determine whether the fill rate is suitable to avoid product foaming or other undesirable artifacts of inappropriate fill times. In some embodiments of the invention, if a weight sensor is faulty, the filling for its nozzle will then be performed solely by time-metered filling (e.g., column 4 lines 23-30).
While these hybrid filling schemes address some of the disadvantages of the individual volume-metered, time-metered, and weight-metered filling methods, they can also combine and compound some of their disadvantages. There is thus still a need for a filling method which provides the accuracy of weight-metered filling, while at the same time avoids its implementation costs and undesirably long filling times.